There are really no difference between themed or not themed. It's just their preferred branding to rely or use the existing games to be their casino's image. What matters is how they balance a great RTP proportionate and mixed well with their House Edge. Those provably fair casinos with themes from the games wouldn't really be as good if they can't have a high RTP and low House Edge, as eventually their players would leave from having their wallets slowly drained not having a balanced win-lose proportion.